[38.03] Europa Cycloids: Key to the Chronology of Tectonics and Implications for Nonsynchronous Rotation

Cycloidal features on Europa appear to form as tensile
cracks in response to diurnal tides of a global ocean [1,2].
Modeling formation of specific features shows where they
likely formed relative to the direction of Jupiter. Imagery
of the southern hemisphere of Europa suggests that Cilicia
and Delphi Flexus are the youngest ridges in the region.
These cycloids are also younger than other prominent
tectonic features including Astypalaea Linea. Modeling of
Cilicia and Delphi Flexus suggest that these features would
have formed as cracks 65\circ and 75\circ to the
west respectively (see also [3]). Cross-cutting
relationships show that a third prominent (but unnamed)
cycloidal ridge is older than Delphi, but still younger than
Astypalaea. Modeling of this feature suggests that it could
have formed 50\circ (modulo 180\circ) to the west.
However the cross-cutting relationships suggest that it
formed before Cilicia and Delphi; thus likely 230\circ
west of its current position.

Tectonic features that are even older according to
stratigraphy (e.g. Astypalaea) must date from previous
cycles of nonsynchronous rotation. That result implies that
an upper limit to the number of cycloids or global scale
lineaments that form regionally over a nonsynchronous
rotation period may be as few as 1 or 2. Moreover, that rate
of crack formation is reasonable , because once a crack
forms the tidal stress is relieved in its region.

This chronology may also be used to constrain formation
rates of ridges and the nonsynchronous rotation period of
Europa. Assuming the lower limit nonsynchronous rotation
period of 12,000 yrs [4], the growth from a crack to the
observed double ridges (100 m high and 2 km wide) associated
with cycloidal features must take place as quickly as 2000
yrs. Alternatively, the fastest possible nonsynchronous
rotation period for Europa would be 170,000 yrs, assuming
estimates of double ridges formation time of 30,000 yrs [5].
What is the slowest possible nonsynchronous rotation period?
Given that only ~1 or 2 cracks form per rotation in the
Astypalaea region, at least 500 rotations would be required
for the tectonic resurfacing that the cratering record
implies happened within the past 108 yrs. Therefore the
nonsynchronous rotation period must be <200,000 yrs, as well
as >12,000 yrs.